In one aspect, this invention relates to the separation of saturated cycloaliphatic hydrocarbons (cycloalkanes, naphthenes) from close-boiling saturated aliphatic hydrocarbons (alkanes, paraffins) by extractive distillation. In another aspect, this invention relates to the use of mixtures of organic compounds as solvent (also referred to as extractant or entrainer) in the aforementioned extractive distillation.
Extractive distillation is a well known technique for separating mixtures of components having a relative volatility close to unity (i.e., having nearly equal volatility and having nearly the same boiling point). It is difficult to separate the components of such mixtures by conventional fractional distillation. In extractive distillation, a solvent is introduced into a distillation column above the entry point of the feed mixture which is to be separated. The solvent affects the volatility of the higher boiling feed component(s) sufficiently to facilitate the separation of the various feed components by distillation and exits with the bottoms fraction, as has been described in the article entitled "Extractive Distillation Saves Energy" by Ian Sucksmith, Chemical Engineering, June 28, 1982, pages 91-95. Other literature sources on extractive distillation techniques include the "Handbook of Separation Techniques for Chemical Engineers" by Philip A. Schweitzer, McGraw-Hill Book Company, 1979, pages 1-135 to 1-143; and Perry's Chemical Engineers Handbook, 6th Edition, McGraw-Hill Book Company, 1984, pages 13-53 to 13-57.
The separation of naphthenes (cycloparaffins), in particular cyclohexane, from close-boiling paraffins by extractive distillation is known and has been described in the patent literature, such as in U.S. Pat. Nos. 2,771,494; 2,891,894; 4,053,369 and 4,921,581. However, there is an ever present need to develop more selective solvents than those presently known in the extractive distillation of mixtures of close-boiling paraffins and naphthenes. In particular, it is highly desirable to develop improved extractive distillation processes for producing cyclohexane of high purity, which is a starting material for making nylon and other useful polymeric materials.